The present invention relates to an exposure method for exposing a resist-coated substrate, and more particularly, to such an exposure method suitable for use in a projection exposure process for printing patterns of various sizes and shapes onto a resist-coated substrate by projection exposure.
In semiconductor device fabrication, a mask having patterns formed thereon is used for transferring the patterns onto a photoresist-coated, photosensitized substrate by projection exposure, and the patterns formed on a single mask typically include line-and-space patterns and single-line patterns. A line-and-space pattern comprises crowded parallel lines extending adjacent to each other, while a single-line pattern comprises one or more lines which have no adjacent line extending close thereto. In general, those lines of the patterns on the mask which have the same linewidth will not necessarily provide the corresponding resist lines of the same linewidth (which are lines corresponding to the mask pattern formed by or in the resist on the substrate after development of the resist) even if the exposure light intensity is uniform over the exposure field.
A mask used in photolithographic process in semiconductor device fabrication typically has several regions defined thereon which provide different interproximity effects. For example, a memory device, such as a dynamic random-access-memory, has regions of crowded lines in which memory cells are spatially-periodically disposed as well as regions of peripheral circuits in which isolated-line patterns are confined. Thus, with the microminiaturization of semiconductor devices, various problems have arisen in this relation including a problem that patterns of different shapes have different space-to-image contrasts and a problem concerning the interproximity effects in which the resist linewidth corresponding to a pattern line is affected by the presence of adjacent pattern lines. These problems have been coped with so far by modifying the linewidth of the pattern lines on a mask for each of different regions on the mask, such that a uniform resist linewidth may be obtained after the development process.
Even within a local region which is homogeneous throughout itself, the resist-linewidth variation may occur between the center and border areas of a crowded-line region since these areas are subject to different interproximity effects. In such case, a certain technique has to be used to compensate for the variation, such as to modify the linewidth of the pattern lines formed on the mask within the border areas or to add compensating patters to the border areas.
Further, even patterns of the same shape and size, which are however located at different positions in the exposure field (say, one is located near the center of the field and the other near the peripheral edge of the field), may produce different resist linewidths due to the image-formation characteristics of the projection lens used. This is because a projection lens typically has image-formation characteristics in which the image quality is poorer at positions further away from the center and nearer to the peripheral edge of the exposure field. In addition, even patterns of the same shape and size and located at adjacent positions may produce considerably different resist linewidths, if they are oriented in different directions. Moreover, any variation in the exposure energy intensity within the exposure field may be a cause of this problem. Specifically, a local area in the exposure field having significantly higher exposure energy intensity is called a xe2x80x9chot spotxe2x80x9d, which may sometimes appears at the center of the exposure field. Such a hot spot causes variations in the resist linewidth. Other factors in the resist linewidth variation include: NA (numerical aperture) of the projection lens used in the projection exposure apparatus; NA of the illumination optical system of the apparatus; the shape of the aperture stop of the illumination optical system such as an annular aperture-stop; the wavelength of the exposure light (such as G-line or I-line from a mercury lamp or other light from an excimer laser); and the characteristics of the photoresist used. Accordingly, in order to eliminate or suppress the resist linewidth variation by modifying the mask, a lot of different masks have to be prepared for various setups of the projection exposure apparatus used and/or for various photoresists selected, resulting in a considerable expenditure for the masks.
In view of the foregoing, it is an object of the present invention to provide an exposure method in which the resist lines obtained after the development process may have the same linewidth or may be equal to the intended linewidths even when patterns which are different in shape and/or size and are subject to different interproximity effects are transferred onto a resist-coated substrate by projection exposure.
It is another object of the present invention to provide a method of fabricating a semiconductor device using such an exposure method.
In accordance with an aspect of the present invention, there is provided an exposure method, comprising the steps of: providing a resist-coated substrate; providing a first mask including first and second regions having respective patterns formed therein; conducting a first exposure process in which the patterns in the first and second regions of the first mask are projected onto the substrate so as to expose first and second regions of the substrate which correspond to the patterns, respectively; and conducting a second exposure process after completion of the first exposure process so as to make an additional exposure of the second region of the substrate.
The second exposure process may comprise the step of making an exposure of the second region of the substrate with light having a substantially uniform exposure intensity throughout the second region of the substrate.
With this exposure method, by virtue of the incorporation of the second exposure process conducted after completion of the first exposure process so as to make an additional exposure of the second region of the substrate, the resist linewidth which would be otherwise produced in the second region of the substrate by the first exposure process can be modified through the second exposure process. Accordingly, the variation in the resist linewidth, which would be otherwise produced by the first exposure process where the patterns in the first and second regions of the first mask are different in shape and/or size and thus are subject to different interproximity effects, can be minimized through the second exposure process.
Further, the second exposure process may comprise the steps of removing away the first mask and providing a second mask having an opaque region corresponding to the first region of the first mask.
In such case, the second mask, which has an opaque region corresponding to the first region of the first mask, is used in place of the first mask so as to make the additional exposure. That is, a bias exposure is effected so that the resist linewidth in the second region of the substrate may be modified.
Moreover, the second exposure process may comprise the step of projecting again the pattern in the second region of the first mask onto the substrate so as to make the additional exposure of the second region of the substrate. In addition, the step of projecting again the pattern in the second region of the first mask onto the substrate may comprise the step of providing a second mask to be overlaid on the first mask, the second mask having an opaque region corresponding to the first region of the first mask. In such case, the additional exposure is effected with light passing the pattern in the second region of the first mask, so that the resist image on the substrate corresponding to that pattern is free from the deterioration of its shape, which could otherwise possibly occur due to the additional exposure.
In accordance with another aspect of the present invention, there is provided an exposure method, comprising the steps of: providing a resist-coated substrate; providing a first mask including a predetermined region having a pattern formed therein; conducting a first exposure process in which the pattern in the predetermined region of the first mask is projected onto the substrate so as to expose a region of the substrate which corresponds to the predetermined region of the first mask; providing a second mask including a partially-opaque region corresponding to the predetermined region of the first mask, the partially-opaque region having transmittance varying depending on the position in the partially-opaque region; and conducting a second exposure process in which the second mask is used to make an exposure of the substrate.
With this exposure method, the second exposure process may be conducted with the first mask removed away after completion of the first exposure process and using the second mask (the second mask includes the partially-opaque region corresponding to the predetermined region of the first mask and the partially-opaque region has transmittance varying depending on the position in the partially-opaque region) so as to make an exposure of the substrate. In this manner, the various resist linewidths which would be otherwise produced in the region of the substrate which corresponds to the predetermined region of the first mask by the first exposure process can be modified depending on the variations in the transmittance of the second mask. Accordingly, the variation in the resist linewidth for the patterns in the predetermined region of the first mask, which would be otherwise produced by the first exposure process where the patterns are different in shape and/or size and thus are subject to different interproximity effects, can be minimized through the second exposure process.
Further with this exposure method, the second exposure process may be conducted with the second mask overlaid on the first mask. In this manner, the deterioration of the shape of the resist image may be minimized.
Moreover, the step of providing the second mask may comprise the step of positioning the partially-opaque region in a plane off a conjugate plane that is conjugate to an exposed surface of the substrate. In this manner, any image of the partially-opaque region can be sufficiently blurred.
In accordance with a further aspect of the present invention, there is provided a method for fabricating a semiconductor device using any of the above exposure methods. With this fabrication method, high-quality semiconductor devices may be fabricated even if a mask having fine patterns differing in shape is used, since the resist linewidth(s) of any resist patterns which are subject to the interproximity effects of adjacent patterns may be compensated so as to provide the intended resist-linewidth(s).